1. Field of the Invention
The present invention relates to a radiation image forming unit comprising a stimulable phosphor sheet and a case housing the stimulable phosphor sheet, and a radiation image forming cassette for housing the stimulable phosphor sheet.
2. Description of the Related Art
There is known a system which employs a stimulable phosphor which, when exposed to radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores a part of the energy of the radiation, and, when subsequently exposed to stimulating light such as visible light, emits light in proportion to the intensity of the stored energy of the radiation. In the system, the radiation image information of a subject such as a human body or the like is reproduced on a photographic photosensitive medium or the like or output as a visible image on a CRT or the like.
The above system has, integrally or individually, an imaging device for temporarily recording (imaging) the radiation image information of a subject such as a human body or the like on a stimulable phosphor sheet which comprises a stimulable phosphor layer (image recording layer) in the form of a sheet, a reading device for applying stimulating light such as a laser beam to the stimulable phosphor sheet to cause the stimulable phosphor sheet to emit light representing the recorded radiation image information and photoelectrically reading the emitted light, and an erasing device for erasing radiation image information that remains on the stimulable phosphor sheet after the emitted light is read.
For handling such a stimulable phosphor sheet with greater ease, there is normally employed a radiation image forming unit having a stimulable phosphor sheet housed in a case such as a magazine, a cassette, or the like. A cassette, known as such a radiation image forming unit, is disclosed in Japanese laid-open patent publication No. 2002-156716, for example.
As shown in FIG. 14 of the accompanying drawings, the disclosed cassette has a front panel 1 and a back panel 2. The back panel 2 includes a back panel body 2a having a plurality of spaced ribs 2b, and a thin plate 3 bonded to the ribs 2b. The back panel 2 also has a lead plate (radiation shield plate) 4 mounted on the thin plate 3, a thin plate 5 fixedly mounted on the lead plate 4, and a support plate 6 replaceably bonded to the thin plate 5 by a double-sided tape or the like. A stimulable phosphor sheet 7 is fixedly mounted on the support plate 6.
The front panel 1 and the back panel 2 can be fixed to each other by a lock mechanism (not shown). The stimulable phosphor sheet 7 is exposed to radiation through the front panel 1. For reading radiation image information recorded on the stimulable phosphor sheet 7, the front panel 1 and the back panel 2 are separated from each other, and the stimulable phosphor sheet 7 and the back panel 2 are placed together in a reading position in a reading device.
The effect that scattered rays have on the stimulable phosphor sheet 7 generally differs depending on imaging applications and conditions. In some cases, no radiation shield plate may be required, and in other cases, a relatively thick radiation shield plate may be required.
According to the disclosure of Japanese laid-open patent publication No. 2002-156716, the lead plate 4 for blocking backward scattered rays is sandwiched between the thin plates 3, 5 and unpeelably fixed to the back panel body 2a. Consequently, the disclosed cassette cannot make itself easily adaptable to different imaging applications and conditions. Even when no radiation shield plate is required, the lead plate 4 fixedly remains sandwiched between the thin plates 3, 5, making the cassette unduly heavy.
There has been known a process for recording the radiation image information of a subject simultaneously on a plurality of stimulable phosphor sheets to perform energy subtraction processing on radiation images. According to the energy subtraction processing, a plurality of radiation images recorded (captured) under different radiation energy absorbing conditions are photoelectrically read to generate digital image signals, and the digital image signals are subjected to a subtraction process at respective pixels of images for obtaining differential signals to extract particular body structures (soft tissues of internal organs, bones, etc.) having different energy absorbing characteristics of a subject whose radiation image has been recorded.
For example, a plate-like filter made of metal (radiation energy separating filter) for absorbing a low radiation energy component is interposed between two stimulable phosphor sheets, and the two stimulable phosphor sheets are simultaneously exposed to radiation that has passed through a subject. The two stimulable phosphor sheets are thus irradiated with radiations having different energy distributions.
In such a process, the two stimulable phosphor sheets and the plate-like filter are stored in a cassette when they are exposed to radiation. After the exposure, the stimulable phosphor sheets are removed from the cassette and placed in the reading device, which then reads the recorded radiation image information from the stimulable phosphor sheets. One such cassette is disclosed in Japanese patent publication No. 8-33596, for example.
Specifically, as shown in FIG. 15 of the accompanying drawings, a cassette 1a that is disclosed in Japanese patent publication No. 8-33596 comprises two stimulable phosphor sheets 7a and a cassette casing 4a for storing the two stimulable phosphor sheets 7a and also a radiation energy conversion filter 3a such as a copper plate disposed between the two stimulable phosphor sheets 7a. The cassette 1a also has a lid 5a openably and closably mounted on the cassette casing 4a. The cassette casing 4a has a slit 6a defined in a side wall thereof for taking the radiation energy conversion filter 3a into and out of the cassette casing 4a therethrough in the directions indicated by the arrow B.
When the cassette 1a is loaded into a loader (not shown) after the stimulable phosphor sheets 7a have been exposed to radiation, the lid 5a is opened, and one of the stimulable phosphor sheets 7a is removed from the cassette casing 4a. Then, the cassette 1a is unloaded from the loader, and the radiation energy conversion filter 3a is removed from the cassette casing 4a through the slit 6a. Thereafter, the cassette 1a is loaded into the loader again, and the second stimulable phosphor sheet 7a is removed from the cassette casing 4a. 
According to the disclosure of Japanese patent publication No. 8-33596, the cassette 1a needs to be unloaded from the loader to remove the radiation energy conversion filter 3a when the radiation image information stored in the second stimulable phosphor sheet 7a is to be read. Consequently, it is relatively tedious and time-consuming to handle the cassette 1a. There has been a demand for a simpler process of handling the cassette 1a. 